In recent years, as an electrode material for non-aqueous electrolyte secondary batteries, a material containing a high capacity element has been attracting interest. For example, a material containing silicon (Si) or tin (Sn) has been attracting interest as a negative electrode active material with high capacity. The theoretical discharge capacity of Si is approximately 4199 mAh/g, which corresponds to an amount approximately 11 times as large as the theoretical discharge capacity of graphite.
However, these active materials, during the absorption of lithium ions, undergo a great change in their structures and expand. As a result, the active material particles break or the active material is peeled off from the current collector. This results in a reduction in electron conductivity between the active material and the current collector, which may degrade battery characteristics (in particular, cycle characteristics).
Under these circumstances, there has been proposed to use an oxide, a nitride, an oxynitride, etc. containing Si or Sn. In active materials including these, although the discharge capacity is slightly reduced, the degrees of expansion and contraction are reduced. Further, there has been proposed to provide an active material layer with a space for relieving the expansion during the absorption of lithium ions (Patent Documents 1 to 3).
Patent document 1 proposes forming an active material layer including columnar particles in a predetermined pattern on a current collector. A photo resist method and plating techniques are employed in forming a negative electrode active material layer. By forming an active material in a columnar state, gaps are created in the active material layer. This relieves the stress due to the expansion and the contraction of the active material, and prevents the destruction of the active material.
Patent Document 2 discloses an electrode including active material particles slanting with respect to a direction normal to a current collector. By slanting the active material particles with respect to a direction normal to the current collector, the stress due to the expansion and the contraction of the active material is relieved, and the destruction and the peeling-off from the current collector of the active material layer can be suppressed. This results in an improvement in battery characteristics such as cycle characteristics.
Patent Document 3 discloses a method for growing active material particles slanting with respect to a direction normal to a current collector of continuous length. The current collector of continuous length is transferred from a feeding roller to a film-forming roller. An element (an active material source) capable of absorbing and desorbing lithium is emitted from a target so as to be incident on the current collector on the film-forming roller. Between the current collector and the target, a mask for shielding the active material source is disposed so that the active material source cannot be incident on the surface of the current collector from a direction perpendicular thereto.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-127561    Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-155958    Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-196970